Systems employing heat exchangers to heat fluids, for example water, reliant on heat from a condenser of a refrigeration vapour compression cycle are known. Swimming pool heaters utilizing heat pumps are an example.
These utilize a heat exchange at the condenser to elevate fluid temperature by a few degrees Celsius each time they pass through. Systems such as this rely on multiple cycles at high (often fixed) flow rates to heat the fluid to the desired temperature. Alternative heat exchanger systems rely on a single pass to heat the fluid to the required temperature. Temperature control in single pass systems is governed by controlling the flow rate of the heated fluid through the heat exchanger. The flow rate in single pass systems is invariably low to allow greater temperature increases in the heated fluid.
An advantage of the single pass heating method is its ability to produce higher outlet temperatures compared with multiple pass methods by utilizing the superheat in the working fluid to “boost” the temperature of the heated fluid stream. One of the side effects of reducing the flow so considerably is that the heat exchanger increases in length due to the reduction in the overall Heat Transfer Coefficient.
Heated fluid temperature control valves that modulate the heated fluid flow to achieve a set heated fluid outlet temperature, in a single pass system, are an accepted way to control heated fluid outlet temperature. Such control means may work acceptably well with a short flow path single pass heat exchanger. However, such means to control heated fluid outlet temperature becomes increasingly less reliable the longer the path of the heated fluid through a heat exchanger.
Heated fluid temperature regulating valves (with sensing element in the heated fluid outlet stream) also known as “thermostatically controlled valves”) are well used and described in prior publications—see for example NZ 250366 and NZ 187940.
The difficulty in using heated fluid outlet temperature, particularly with a long flow path heat exchanger is that it takes time (sometimes a matter of minutes) for the heated fluid to travel the length of the exchanger, this makes it difficult for a feedback control system to respond to rapid changes in heated fluid inlet temperature and/or system conditions. Hence the use of heated fluid temperature control valves (thermostatically controlled valves), as described in the prior art NZ 250366 and NZ 187940 in longer flow path heat exchangers may result in the flow being subject to wide fluctuations, especially during the start-up phase of the refrigeration vapour compression cycle and during temperature and/or flow fluctuations in the system.
Water flow regulating valves using refrigerant pressure acting against a spring to control valve opening position for the purposes of controlling outlet water temperature are also known in the art (eg Robinson U.S. Pat. No. 4,330,309). Use of these valves to replace thermostatically controlled valves generally results in a much better control characteristic, although there are limitations for modern use. Limitations include the capital cost of the component, integration costs, lack of compatibility with electronic controls, inefficient use of energy and so forth.
It is an object of the present invention to provide a fluid heater and control system and related method for heat exchange of a refrigeration vapour compression cycle system with reduced fluctuations in the heated fluid outlet supply temperature and high efficiency and compared with methods known in the prior art.